State-of-the-art mailing machines can perform such automatic functions as handling mail of different sizes and thicknesses, envelope sealing, mail weighing, mail stamping, and mail sorting. In developing machines with such functions, capable of processing mail at high speeds of, for example, four or more pieces per second, it becomes important if not essential that the mail thickness is determined as soon as possible after the mail begins its flow sequence. Knowing the thickness early is important because there usually is a relationship between mail thickness and mail weight, i.e., the thicker the mail, the more it weighs. Typically, heavier mail must be processed slower than lighter mail in a high speed processing environment. Hence, the weight of the mail allows the computer which is controlling the machine to slow the transport mechanisms when carrying heavy mail and speed up the transport mechanisms when carrying lighter mail.
It is desirable to control transport velocity as a function of mail weight or mail thickness as soon as the mail pieces begin their flow through the machine. Typically, the mail pieces enter the system from a hopper is stacked form, and one of the first actions necessary is to separate an individual piece of mail from the stack. The mehanism for doing this is called a singulator and the action singulating. The copending application Ser. No. 291,098, describes one form of singulator mechanism for use in a high speed mailing machine, the contents of which application are herein incorporated by reference.
Mail thickness sensors in prior art machines would typically position on top of the mail piece a follower connected to an optical system involving a light source scanning across an array of light detectors, the position of the light source being determined by the position of the follower, and the position of the light source determining which detector is activated. Mechanical systems have also been used.
These prior art systems suffer from one or more of the following shortcomings. With optical systems, frequent maintenance is necessary to keep the optics clean. A mailing machine processing thousands of pieces of mail daily does not provide a clean environment for optical sensors. The signal output frequently was analog. This meant the use of an A/D converter to translate the analog signal into a digital signal that the computer can process, which increased costs. Accuracy of thickness measurement was not always optimal. Especially with high speed processing, it is important to be able to measure the mail thickness in the range of 0.004-0.75 inches to an accuracy of about 0.05 inches.